Fix button color in IPFS interstitial page in light theme

[yrliou]

Resolves brave/brave-browser#13599

Buttons in other interstitial pages are using this, this PR adds this for .ipfs button in our own css.
https://source.chromium.org/chromium/chromium/src/+/master:components/security_interstitials/core/common/resources/interstitial_common.css;l=33?q=interstitial_common&ss=chromium

With this, the page looks like below:

Submitter Checklist:

• There is a ticket for my issue.
• Used Github auto-closing keywords in the commit message.
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed).
• Requested a security/privacy review as needed.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

Pre-condition: In brave://settings, Method to resolve IPFS resources is set to Local Node, IPFS public gateway fallback switch is off.

1. Turn off wifi, open brave://ipfs to start the local node with 0 connected peers.
2. Visit ipfs://QmbWqxBEKC3P8tqsKc98xmWNzrzDtRLMiMPL8wBuTGsMnR to see the interstitial page.

[yrliou]

Verified locally, didn't wait for CI to finish as it's just a trivial css change.

[kjozwiak]

Verification PASSED on macOS 11.1 x64 using the following build:

Brave | 1.21.7 Chromium: 88.0.4324.87 (Official Build) nightly (x86_64)
--- | ---
Revision | dd01ff8f58c65af81127ad5c105c79d5b571d8f3-refs/branch-heads/4324@{#1702}
OS | macOS Version 11.1 (Build 20C69)

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